The invention relates to a vertical field type MRI apparatus for forming magnetic resonance images, including:                at least one field generating superconducting coil system for producing a substantially homogeneous magnetic field in an imaging volume of the apparatus,                    which coil system includes:                            a round outer coil that is situated in an outer coil plane;                a round supplementary coil that is situated within the outer coil.                                                
An apparatus of this kind is known from United States patent U.S. Pat. No. 5,939,962. The homogeneous magnetic field required for MR imaging in such a vertical field type apparatus is usually generated by two oppositely situated magnetic poles wherebetween the patient to be examined can be arranged. Generally speaking, said magnetic field then has a vertical direction. Apparatus of this kind offers the advantage that the patient keeps a comparatively broad view of the surroundings when arranged in such an apparatus, so that sensations of claustrophobia occur less frequently.
An iron circuit that is capable of transporting the complete flux through the system becomes very heavy in the case of magnet systems having a field strength beyond approximately 0.5 T. A sensible alternative in that case is to omit the iron circuit completely and to construct the magnet system as an actively shielded air coil system. In that case there are no poles in the sense of iron structures that bound the space of the magnet system that is accessible to the patient, but the surfaces of the magnet system that bound the patient space will also be referred to hereinafter as “poles” for the sake of simplicity. For field strengths beyond 0.5 T the coils must be constructed so as to be superconducting. They are kept at the operating temperature in a cryostat. The “poles” are then formed by the outer wall of the vacuum envelope of the cryostat.
The cited United States patent discloses a superconducting coil system which consists of a round outer coil (a so-called “side coil” that is denoted by the reference 12a therein), a round supplementary coil (referred to therein as the “fourth coil” which bears the reference 12d), and a number of further coils (referred to as the “second and the third coils” bearing the references 12b and 12c therein). The homogeneous field in the imaging volume is generated mainly by the first two coils 12a and 12b and the other coils mentioned superpose a further homogenizing field thereon.
As is generally known, and also described in the cited US patent, for this type of apparatus the aim is to arrange the field generating coil in the upper magnetic pole at an as small as possible distance from the field generating coil in the lower magnetic pole. This aim stems from the fact that the production costs of such a system increase by approximately a power of five of said pole distance, so that it is advantageous to keep this distance as small as possible. Because of this aim, the outer coils in the known apparatus are arranged practically directly against the boundary of the freely accessible space between the magnetic poles.
When the outer coils are mounted in that manner, the gradient coils in the known apparatus must extend to practically the diameter of the associated outer coils because of the necessary linearity of the gradient field in the imaging volume. Consequently, room for the supplementary coil (also having a voluminous and heavy construction so as to achieve the required homogeneous field) can be found only above the upper gradient coil and below the lower gradient coil. Consequently, the construction of this already bulky and heavy coil must be even larger; however, the outer coil must then also become larger. Moreover, in the case of actively shielded magnetic coils the shielding coils must then also become larger. The ultimate effect of the foregoing is that the costs of the apparatus are substantially increased again.